Find out how you are losing coolant. There are only 2 places where the coolant could go: Outside or mixing with engine oil.
Outside means it is leaking somewhere... check the radiator, the hose, the water pump, the reservoir due to coolant being pushed out the radiator and into the reservoir which is overflowing...usually caused by excessive pressure in the system or a bad radiator cap. Most of the time its the latter but you said you've changed it so if you're losing coolant thru the reservoir then its because of too much pressure...usually thats caused by trapped air in the cooling system, head gasket failure or cracked head. Although the last 2 do happen it is a rare thing. Usually it is trapped air which means you have to bleed the system. Very easy to do. Not sure you have a v6 or a v8. I don't know how to bleed the system on the Alloytec but on the Echotec there is a little screw/valve/bleeder on the thermostat housing which you unscrew to release air. On the v8 you disconnect the top radiator hose and fill the system with coolant from there. So for the Alloytec you will have to find out by asking members here or running a search on Google.
If its not a leak then its mixing with oil and if that's the case then it is easy to find out by looking at the oil on the dip stick. If it looks like chocolate milk or dark brown then that is where the coolant is going. That means you either got bad head gasket or cracked head or cracked engine block. Its rarely cracked head or block and mostly just head gasket that needs to be replaced. Before you start replacing the head gasket, you should do a compression test to be sure.
So if I were you I would do the following:
1. Check the oil and see if there is coolant there.
2. Bleed the system. See if the situation changes. If not then...
3. Thoroughly check and see where the leak is. If I find the leak then a fix it depending on where it is...fill the system with coolant and bleed the it.
There shouldn't be any other things to do because if its like you said that you are losing coolant then like I said it is either mixing with oil or escaping out.
Let us know how it goes.
Good luck!
